Opossums are a virtual museum of internal parasites.  Opossums go about their lives unaffected by their parasite load unless some trauma or other inequity gives their parasites an upper hand, and then the opossum may succumb.  Because it is very easy to find many parasites in an opossum, sometimes scientists look no further than blaming the opossum whenever another species is affected by a particular parasite.

Recently California sea otters have been dying off and a brain parasite Sarcocystis neurona, which is shed in opossum feces, has been discovered in the sea otters during necropsy.  California Department of Fish & Game (CDFG) recommended that opossums not be rehabilitated and released in California because of their possible impact on otters.  CDFG did not go so far as to outlaw opossum rehabilitation and are now considering there may be more disease, more contaminants and/or more parasites involved.


three parasites, including one carried by outdoor cats, are being blamed for an outbreak of an infectious disease that is killing California sea otters, according to researchers at the University of California at Davis and the state Department of Fish and Game.

The most serious parasite is Toxoplasma gondii, which causes a brain infection. Researchers say they believe that Toxoplasma spreads as outdoor cats’ feces, laden with millions of parasite eggs, wash into the ocean from streams and drainage ditches.  These hardy eggs, or oocysts, are thought to be absorbed by filter-feeding shellfish that are otters’ prey.

From 1997 to 2001, Dr. Melissa Miller, a pathologist at the University of California at Davis, her partners there, CDFG, the Morro Bay Foundation and the Central Coast Regional Water Quality Board examined 233 live and dead sea otters. They found startling rates of parasitic infections, particularly Toxoplasma.

”Of live, presumably healthy, otters,” Dr. Miller said, ”42 percent had antibodies to Toxo. Of dead otters, 62 percent had antibodies.” The findings were intriguing and troubling, Dr. Miller said, because a disease known only in land mammals had spread to marine mammals.

If an otter has antibodies to Toxoplasma, Dr. Miller said, that generally means a chronic, persistent infection. Toxoplasma and Sarcocystis can create cysts in otters’ brains. The resulting infections can cause seizures or paralysis. Researchers also say infected otters may become disoriented and swim far offshore, where sharks can focus on their twitching bodies. 

In 2000, Dr. Christine Kreuder, a wildlife veterinarian who is a colleague of Dr. Miller’s, started a mortality study as part of her doctorate in epidemiology. With an epidemiologist from the Wildlife Health Center, Dr. Jonna A. Mazet, and other Fish and Game pathologists, they found that 16 percent of all California otter deaths were linked to brain infections caused by Toxoplasma.   Sarcocystis neurona killed 7 percent, and a seabird-borne intestinal parasite called Acanthocephalan peritonitis, or thorny-headed worm, killed 16 percent.

Acanthocephalans typically have complex life cycles, involving a number of hosts, including invertebratesfishesamphibiansbirds, and mammals.

Cats play a major role in the Toxoplasma parasite’s success. They can carry it in their intestines, where they can produce egglike cysts called oocysts. A single infected cat can shed 100 million oocysts in its droppings. The oocysts can survive in the soil for over a year and can contaminate drinking water. 

In 1997, American Bird Conservancy (ABC) launched the Cats Indoors! Campaign for Safer Birds and Cats to educate cat owners, decision makers, and the general public that cats, wildlife and people all benefit when cats are kept indoors.  Visit their website which has pertinent literature that is downloadable  


So even though the Sarcocystis neurona parasite that an opossum carries was linked to 7% of the deaths, Toxoplasma shed by cats and Acanthocephalan parasites shed by many species, including birds, made up for 32% of the deaths and 61% of otter deaths were unexplained.  Why is everyone so quick to blame the opossum for spreading disease when 93% of the otter deaths were attributed to other species/causes?  Why aren’t veterinarian hospitals plastered with literature on keeping cats indoors and not flushing kitty litter down the toilet?  Why hasn’t the California Department of Fish & Game suggested that birds not be rehabilitated because of their impact on otters?  

The opossum needs an advocate; that’s why the Opossum Society of the United States exists.Leslie Bale